Electric wave amplifier



Patented June 2, 1942-' ELECTRIC WAVEv `AMPLIFIER ,Michael J. Burger,South ,Ozone Park, N. Y., assignor to Western yElectr-,ic Company,Incorporated, a corporation of New YorkV Application December 4, 1940,Serial No. 368,416

` a claims. (ci. 11s-111) The invention relates to electric waveamplifiers and in particular to the operation of such y amplifiers so asto reduce or eliminate undesired frequency components in the outputcurrent.

l An object of the invention is to sec'ure efficient amplification ofapplied Waves while reducing or suppressing in the output current theeven order distortion products. tion is the generation in one element ofthe amplier of even order distortion products of such magnitude andphase relationship that they substantially neutralize even orderdistortion products produced in another element of the amplier.

The invention is applicable to electric wave amplifiers for any purpose,but is particularly advantageous'in amplifiers for Waves in the audiofrequency range so that, by the neutralization of the harmonicdistortion, the reproduced audio frequency currents are free fromdistortion.

The invention may conveniently be embodied in the known-amplifier systemcomprising a driver amplifier, and a phase inverter amplifier,conjointly Acontrolling two amplifiers connected in push-pull or opposedrelationship though it vis not limited to this particular use. Inaccordance with the present invention, the circuit ele- 1 mentsassociated with the inverter` amplifier are so proportioned that theeven order distortion products generated in the inverter amplifier are yof such magnitude and phase as to neutralize in the input of thepush-pull amplifier the even order distortion products produced by thedriver amplifier.

A more complete understanding of the invention may be had from thefollowing detailed description,` together 'with the attached drawingwhich is a schematic circuit diagram of a typical embodiment of theinvention.

Electric waves from any desired source are apthe capacitor 1 to producea voltage across the A feature of the inven to produce a voltage acrossthe resistor I4 connected across the input circuit of the amplifier I5forming the other side ofthe push-pull cornbination.` The voutputs ofthe push-pull amplifiers IU and I5 are supplied in the usual manner toseparate primary windings of the transformer IB, and thence throughtransformer IB to the output circuit. The driver amplifier 5 may beself-biased by the usual cathode resistor I'l, and

similarly the inverter amplifier II may be selfbiased by the cathoderesistor I8. The cathode bias resistors may be shunted by the usual bypas condensers. Power from any suitable sources is supplied to thecathodes and anodes of the various amplifiers in the usual manner.

Assume that an electric wave of fundamental frequency -I-F' is appliedto the input circuit of the driver amplifier 5 of sufficient magnitudeto heavily load the driver amplifier. The output circuit of the driveramplifier 5 will then contain the fundamental frequency -F and the evenorder distortion products, which may be represented by +F2,. and theseare applied to the input circuit of the push-pull amplifier I0. Aportion of the output vof the driver amplifier w-F and -I-F2 is alsosupplied to the input circuit of the inverter amplifier II. Inaccordance with the present invention the circuit elements associatedwith the inverter amplifier II are so proportioned that the inverteramplifier II produces a second order distortion product which has twicethe amplitude of the second order distortion'product y produced 'by thedriver amplifier 5, vand an amplication such that the output at thefundamental will be equal in magnitude to the output v of the driverstage. The output .of the inverter amplifier Il will comprise the inputinverted in phaseand the generated second order distortion products, andwill thus comprise -|2F2,V F2 and t-FZ, which is equal to -I-F-l-FZ. Theoutput of the inverter amplifier II will be -l-F-i-FZ, where'- as, theoutput of the driver amplifier 5 Will be F4-F2. In accordance with thewell-known operation of push-pull circuits, the fundamental age acrosstheresistor 6 which is applied through resistors and 9 connected inserial 'relationship across the input circuit of the amplifier I0,forming one side of a push-pull amplifying arrangement. The voltagedeveloped across the resistor v9 is also supplied to the input circuitof the inverter amplifier Il. The output of the inverter amplifier IIproduces a voltage across the resistor I2, which is applied through thecapacitor I3 frequency F supplied to the input circuit-ofr the amplifierI0 and the fundamental frequency +F supplied to the input circuit of theamplifier I5 will be combined in the primary windings of the transformerI6 and supplied through the transformer I6 to any desired load orutilization circuit. On the other hand, the second order distortionproduct in the output of the driver amplier 5 and the similar 'secondorder distortion product-in the output of the inverter amplifier II willmutually neutralize each other in the input type of vacuum tubeselected, the grid bias and anode voltages for the driver amplifier areselectwhereas C1 was .1 microfarad. Under these coned so that when'thedriver amplifier is fully loaded the output voltage applied to theamplifier I will load the amplifier I0 to a desired maximum value. Underthese conditions, the driver amplifier 5 will have a known impedancebetween anode and cathode and should operate into an output circuithaving a preferred value of alter-` l5 'nating current impedance whichis determined even order distortion products will be 6 decibels greaterthan the output of even order distortion products from the driveramplifier 5. The relative values of the resistors 8 and 9 are thenselected so that the output voltage from the inverter amplifier IIapplied to the push-pull amplifier I5 will be equal to the outputvoltage from the driver amplifier 5 applied to the push-pull amplifierl0.

In a practical embodiment of the invention, the driver amplifier 5 andthe inverter amplifier II were Western Electric 262-B vacuum tubes andthe amplifiers III and I5 were Western Electric 30D-A vacuum tubes. Theresistor 6 was 150,000 ohms, the resistor 8 was 400,000 and the resistor9 was 40,000 ohms,-the biasing resistor I1 being `7,500 ohms. Theresistor I2 was 125,000 ohms, the resistor I4 was 300,000 ohms, whilethe biasing resistor I0 was 6,500 ohms. `Neglecting the,4 couplingcapacitors 1 and I3, and assuming input impedance ofthe amplifiers I0and I5 to 'be infinite, the driver amplifier 5 was working into analternating current load of some 112,000 ohms whereas the inverteramplifiers II was workinginto an alternating current load impedance ofsome 88,000 ohms. It should be noted that in order to maintain theproper phase inversion relationship that capacitor I3 must be madeconsiderably larger than capacitor 1. In this typical embodiment CIS was.25 microfarad ditions, the even order distortion products in the outputof the push-pull amplifier were reduced some 20 decibels below the valuethat these products would have if the driver amplifier 5 and theinverter amplifier II were operating under the same conditions, and wererespectively associated with circuit elements of the same magnitude.

-What is claimed is:

1. A push-pull amplifier, a driver amplifier for supplying oscillationsto one side of said pushpull amplifier and having circuit elements ofsuch constants as to produce a definite ratio of even order distortionproducts to the fundamental of the applied oscillations, an inverteramplifier having aninput connected to the `output of said driveramplifier and an output connected to the other side of said push-pullamplifier and having circuit elements of such constants asl to produceeven order distortion products of the applied oscillations of suchmagnitude and phase as to substantially neutralize the even orderdistortion products supplied by said driver amplifierv to said push-pullamplifier.

2. A resistance-capacitance coupled amplifier ccmprisinga driveramplifier, an inverter amplifier, and two amplifiers in push-pullrelationship, characterized by the circuit elements associated with theinverter amplifier being so proportioned that the inverter amplifierproduces even order distortion products of the applied signal havingtwice the amplitude of the even order distortion products produced bythe driver tube, whereby the even order distortion products supplied bythe inverter amplifier to the push-pull amplifiers substantially cancelthe even order distortion products supplied by the driver amplifier tosaid push-pull amplifiers.

3. In combination, a load circuit, a first amplifier for supplyingoscillations to said load circuit having circuit elements of suchconstants as to produce a definite ratio of even order distortionproducts to the fundamental frequency of the applied oscillations, and asecond amplier having an input coupled to the output of said firstamplifier for supplying oscillations equal in magnitude and opposed inphase to the oscillation from said first amplifier to said load circuit,having circuit elements of such constants as to produce reven orderdistortion prod cts of the funy NncrLAEL J. BURGER.

